In refiners of the drum type, to which this invention relates, the paste-like grist to be refined between the working surfaces of the drum and the wings is propelled in wave-like fashion in rapid pulsation succession while being subjected to a wedging action as it is accelerated by the centrifugal force exerted by the wings, which normally rotate at a linear speed ranging between 15 and 100 meters per second along the interior surface of the drum. These wings exert a force in a linear direction on the grist as it is pushed along by the leading wind edges, which force might be on the order of 5 Kg. to 10 Kg. per square centimeter, causing the grist to become compacted into wedge-shaped clumps of such a density that friction forces are induced within the grist clumps. The induced forces rise to such intensity that a plane of shear is created a short distance from the leading edge of the wings so that the wedge-shaped fiber bundles are not only broken up, but the primary layer of tracheide is substantially rubbed off, leaving the secondary layer exposed, with consequent improved fiber-to-fiber adhesion by the hydrogen bonds in the finished paper. Heretofore, this breaking up of the fiber bundles and the subsequent fibrillation of the fibers has been carried out at a fixed distance between the terminal edges of the wings and the shear members without any substantial direct contact between the individual fibers and the metallic shearing surfaces.
It should be understood that the fibers have a diameter of only some hundredths of a millimeter. Therefore, in disc refiners, or disc grinders, the spacing between the grinding elements must be extremely narrow, such as some tenths of a millimeter, so that these hair-like fibers can be firmly gripped between the grinding surfaces, with consequent risk of increased wear of and damage to the grinding elements. This risk is substantially avoided by the drum type refiner, to which the present invention relates, in which the gap between edges of the wings and the shear members may range between one mm. and two mm., while still producing a satisfactory shearing action and fibrillation of the fibers.